The first sentence is always the hardest. I normally have a character make a comment

Skip the opening and start writing the middle. You don't always need to start from the beginning. With that said, the first sentence needs to catch people's attention; not just to hook people into reading the story, but more importantly, people will skip past the introduction if it seems boring. Start with some sort of action, be it speech or an actual act; don't start with a description of something, unless that something is incredibly awesome and you can make it sound really compelling.

There's no sure fire way to start, it tends to be different for everyone. I usually start with a thought the character is having.

Starting with a quote helps.

Or, like others have suggested, start in the middle, right in the thick of the action. Beginnings are hard, so warm up with something easier first.